In 1996, the central fuel tank in a Boeing 747, flying as TWA Flight 800 from New York City to Paris, exploded a few minutes after takeoff, and the airplane broke apart and fell into the sea close to the southern shore of Long Island, with all lives lost. The cause of the disaster was eventually attributed to a spark in the central fuel tank, issuing from electrical wiring for the fuel metering gauge, whose insulation had cracked over time.
At the time, I wondered if the explosion could have been a natural phenomenon, basically could lightning have been generated inside the fuel tank while the airliner was climbing from sea level to 8000 feet (2400 m)? I spent a great deal of time during the next ten years working on a physics model that might answer that question. By 2006, the National Transportation Safety Board (NTSB) had arrived at its conclusions about TWA 800, and new rules for the installation of aviation fuel tank flammability reduction (“inerting”) systems were being issued for US carriers.
Also in 2006, Airbus Industries was introducing the A380, its massive double-decker passenger transport. This airplane was built to European safety standards, which did not call for active fuel tank flammability reduction systems. The NTSB and Federal Aviation Administration (FAA) tried to influence their European counterparts to follow the American lead on the issue of fuel tank safety, but the Europeans resisted. The Europeans believed the design of the A380’s fuel tanks was sufficiently different from those of the much older Boeing 747s and 737s, so that the A380 would not experience the same catastrophe. It was also obvious that adding fuel tank flammability reduction systems decreased an airplane’s fuel efficiency (by increasing weight) and profitability (by increasing the purchase price and decreasing the maximum payload).
However, if natural “lightning in a tank” was physically possible and the A380’s fuel tanks were sufficiently large for this phenomenon to take place in them, then even the elimination of interior wiring for fuel gauges would not eliminate the hazard of a spontaneous fuel tank explosion in the A380.
I was never able to quantify the physics sufficiently to specify if and when “lightning in a tank” would occur, and I never found publicly available information of the designs and sizes of A380 fuel tanks, so the questions posed remain open. To be clear and avoid being alarmist, let me say that both the quantitative implications from my physics theorizing and the evident fuel tank safety record of the airline industry since 1996 both show that natural “lightning in a tank” is extremely unlikely, and it may well be impossible.
The best way to know for sure would be to do an experiment duplicating the climb to altitude by TWA 800. Subject an instrumented duplicate of the TWA 800 central fuel tank with the same fuel-air-water fill, and at the same initially heated state, to a simulated climb that matches the dynamic conditions of TWA 800: the nonuniform heating and cooling of the outer walls, the atmospheric pressure changes, and the same degree of mechanical agitation. Measurements of the amount of spray electrification inside the tank would then show how strong of an electrical effect can be generated by an exterior combination of thermodynamic and mechanical energy flows into the tank walls (and vent pipe).
I wrote a report for a general audience in 2006 describing the overall picture of what I had learned about airliner fuel tank explosions, and the corresponding safety regulations at that time. It was turned down for Internet publication because of fears that it might be too alarming without provable basis. I don’t think the article was unreasonable, you can judge for yourself.
The information in the article, about commercial passenger aviation fuel tank safety regulations and practices, was accurate as of the beginning of 2007. I have not followed developments in the regulatory arena since then.
The good news is that economical flammability reduction systems were developed (even by 2006, as described in the article), and their deployment, both as upgrades to older airliners and original equipment in new ones, is a major safety enhancement for the commercial passenger aviation industry.
manuelgarciajr 